1. Field of the Invention
The present invention relates to a tracking mechanism and method for an apparatus which records/reproduces information on/from a medium by scanning the medium with probes. The apparatus may be a high density, high capacity memory device using the STM principle.
2. Related Background Art
The advent of recent scanning tunneling microscopes (hereinafter abbreviated as STM) capable of directly observing an electron structure of a conductor has realized high resolution measurements of a real spatial image irrespective of whether the conductor is single crystal or polycrystal (G. Binning et al. Phys. Rev. Lett. 49, 57 (1982)). After this advent, scanning probe microscopes (SPM) and fine patterning techniques using SPM have been studied and developed. SPM can obtain various pieces of information by scanning a subject with a probe having a sharp tip. Fine patterning techniques using SPM aim to electrically, chemically, or physically process the surface of a substrate. Such SPM techniques are being applied also to memory techniques. For example, JP-A-63-161552, JP-A-63-161553, and other publications disclose a method of recording/reproducing information in/from a thin recording film by using STM. The thin recording film is made of a material having a function of memorizing a voltage-current switching characteristic, such as .pi. electron based organic compound and chalcogen compound. With this method, an information processing apparatus having a record density of 10.sup.12 bits/cm.sup.2 can be realized at a record bit diameter of 10 nm.
In reading information recorded on a medium, an information reading probe is required to be moved relative to a data bit train on the medium. For the relative motion of the probe, it is necessary to know the position of the data bit train and move the probe to this position.
As a method of detecting the position of a data bit train, one method is known in which physical tracks are formed on a medium and a probe is moved along the track.
JP-A-1-107341 discloses a method of forming a V-character shaped groove on the surface of a recording medium as a track, and controlling a probe electrode to be always at the center of this groove.
JP-A-1-133239 discloses a method of forming a conductive layer in a recording medium as a track and applying a tracking signal to the track to perform a feedback control by using the tracking signal detected with a probe.
As a method of tracking a data bit train without forming a track on a recording medium, JP-A-4-212737 discloses a tracking method in which a probe is vibrated in a direction perpendicular to a data bit train at a small amplitude to detect the position of the bit train and prevent position misalignment.
An approach to making the tracking control unnecessary without using the feedback control of position misalignment has been proposed in which the area near a data bit train is scanned at some density to thereby read the information.
For example, JP-A-2-050333 discloses a method of reading information by scanning a medium with a probe at a density higher than a recording density and by using pattern recognition techniques.
JP-A-4-355231 discloses a method of reproducing information in which two preliminary scans are performed before the information is actually read, tunneling current at each scan is integrated to obtain an output signal value, a center position of a signal train is detected in accordance with the output signal values at the first and second preliminary scans, and the information is reproduced by a third scan along this detected center position.
The method of forming physical tracks on a medium as disclosed in JP-A-1-107341 and JP-A-1-133239 is, however, associated with some problems that an additional process of forming physical tracks makes complicate the process of forming a recording medium and that technical and economical issues arise from required high working precision.
A method disclosed in JP-A-4-212737 is also associated with a problem that a scan speed is limited by the maximum frequency of fine vibrations because the frequency of fine vibrations in a direction perpendicular to a data bit train is required to be faster than the scan speed.
A method using pattern recognition techniques disclosed in JP-A-2-050333 is also associated with a problem that it takes a long scan time to read information at a high density, pattern recognition at a post process is complicate, and a process time is additionally required.
The methods disclosed in JP-A-4-364244 and JP-A-4-355231 are also associated with a problem of a long scan time because three scans are used for reading one data bit train.